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Kayo once again needs to borrow money in order to buy a Christmas present, and Mamie is kind enough to be the first to contribute, even though the gift is for her. Perhaps she thought that the residents of the boarding house and the surrounding city would be as kind as the people that work in Smitty's office. As we see, this is not necessarily true.
For those unfamiliar with Moon Mullins, Willie and Mamie are Moon and Kayo's aunt and uncle, and Mamie works for the Plushbottoms, owners of the boarding house, as a washerwoman. Willie... doesn't work, and is portrayed as lazy and shiftless. He also seems to have a magnetic pull towards anyone with small amounts of cash. As soon as Kayo walked outside with that quarter, Willie pounced on him and convinced him to give it up. I guess he figured it would be easier than asking his wife.